Switched mode power converters, in particular but not limited to AC-DC converters, typically employ feedback from the secondary to the primary side in order to control the power of the converter. For many applications, however, electrical isolation is required between the two sides, typically in order to protect the user on the secondary side from relatively high voltages on the primary side (such as mains voltages). For such applications, an optocoupler is a preferred means to provide the feedback across the isolation, since feedback information is transferred from the electrical domain to an optical domain and back.
In known feedback systems, the optocoupler is used to transfer an error signal (Vout−Voutref), indicative of the difference between the actual output voltage (Vout) and the desired output voltage (Voutref). The feedback current on the output side of the optocoupler (that is to say on the primary side of the converter), increases typically linearly with the error signal, as shown schematically in FIG. 1, which shows a plot of the output current (Iopto) 10 from the optocoupler, against the converter output voltage (Vout). The feedback current Iopto is typically directly related to the input current (Ioptin) driving the optocoupler (that is to say, on the secondary side of the optocoupler)—for instance, the feedback current Iopto may be directly proportional to the input current (Ioptin), through a gain factor g. In some implementations, for converter voltages below or equal to Voutref, the feedback current is zero, whilst above Voutref the current increases linearly with the error signal (Vout−Voutref). Further, the error loop may include frequency-dependant elements for implementing integrating and/or differentiating actions.
European patent application EPA1,648080 discloses a controller in which a feedback signal rises in inverse proportion to the output voltage.
Under low-power, or no load, conditions the current required by the optocoupler to feedback the error signal can be relatively high. In particular to increase converter efficiencies at low load, there is an ongoing desire to reduce the power consumption in the feedback mechanism.